Photoluminescence is often referred to as fluorescence and fluorescence is the emission of light from fluorescent materials. Fluorescent materials contain luminescent atoms or molecules, which can go into an excited state by absorbing electromagnetic erergy from an external source, which can come in the form of light. This electromagnetic energy, after being absorbed, is then released by the luminescent atoms or molecules, in the form of light. The emission of light from a fluorescent material is thus known as fluorescence. The imaging of latent fingerprints through fluorescence is one use of this phenomenon.
Whenever a finger touches a surface or substrate, it leaves a residue behind. This residue is mainly composed of water, which dries up eventually, leaving behind both inorganic salts and organic constituents. The pattern of deposition of this residue forms distinctive fingerprints that are unique to each individual. As some of these compounds are fluorescent, forensic scientists can use photoluminescent techniques to visualise fingerprints. Laser Induced Fluorescence (LIF) detection of fingerprints was demonstrated Dalrymple et alis in 1976 (Dalrymple et al “Inherent fingerprint luminescence—detection by laser,” Journal of Forensic Sciences, Vol. 21, 1976, pp: 106–115). In that technique, optical filters were used to differentiate the fingerprint fluorescence from the substrate fluorescence, on the differences in intensity and colour of the fingerprint and substrate fluorescence. However, LIF detection of fingerprints using optical filters is fraught with problems when the substrate material fluoresce at a colour similar to that of the fingerprint fluorescence.
To overcome this problem, a new method uses the lifetime property of fluorescence to differentiate between the fingerprint and substrate fluorescence (Menzel, 2001). This method, the time resolved method, differentiates between the fingerprint fluorescence and the substrate fluorescence based on the lifetimes of each fluorescence source as opposed to simply differentiating between the colour and intensity.
As such, the time resolved method is able to image fingerprints that were once difficult to visualise. However, the time resolved method has two main limitations. This technique requires a large difference in fluorescence lifetimes between the different fluorescences in order to filter one fluorescence from the other. Furthermore, in order for time resolved method to work, the fingerprint fluorescence lifetime has to be longer than that of the substrate. Consequently, the time resolved method is only able to image fingerprints from a restricted range of substrates.
A technique that is able to discern fingerprints from a larger range of substrates is the phase resolved method, which makes use of the phase difference between the fluorescence of the fingerprint and the substrate when excited by a modulated illumination source. The advantage of using phase resolved method for fingerprint detection is that the fluorescence lifetime is no longer a consideration. Although phase resolved method is now being used in applications such as fluorescence microscopy and imaging of thick tissue, it has not been used in latent fingerprint detection due to the difficulties encountered.
The phase resolved method requires the fingerprint and the substrate to be excited to fluoresce by an illumination source that is modulated at a high frequency. This high frequency modulation, which can be of the order of several ten megahertz (MHz), is required in order to obtain a measurable phase difference to successfully suppress substrate fluorescence. However, it is difficult to carry out signal processing at such high frequencies since the image capturing devices are unable to respond to such high frequencies.
Another difficulty encountered in the use of the phase-resolved method for imaging fingerprints is that fingerprint fluorescence is too weak to be effectively detected by common detectors. As the substrate fluorescence may be strong compared to the fingerprint fluorescence, it can be difficult to suppress unwanted fluorescence components when imaging fingerprint samples. Therefore, although phase resolved method has proven useful in other applications, this method is not used in fingerprint fluorescence imaging as this method is currently unable to consistently obtain clear images of fingerprints from fingerprint samples.